Add Bitlist and Bitvector

specs/core/1_custody-game.md

@@ -272,6 +272,16 @@ def get_custody_chunk_count(crosslink: Crosslink) -> int:
     return crosslink_length * chunks_per_epoch
 ```
 
+### `get_bitfield_bit`
+
+```python
+def get_bitfield_bit(bitfield: bytes, i: int) -> int:
+    """
+    Extract the bit in ``bitfield`` at position ``i``.
+    """
+    return (bitfield[i // 8] >> (i % 8)) % 2
+```
+
 ### `get_custody_chunk_bit`
 
 ```python
@@ -566,7 +576,7 @@ def process_bit_challenge(state: BeaconState,
     chunk_count = get_custody_chunk_count(attestation.data.crosslink)
     assert verify_bitfield(challenge.chunk_bits, chunk_count)
     # Verify the first bit of the hash of the chunk bits does not equal the custody bit
-    custody_bit = get_bitfield_bit(attestation.custody_bitfield, attesters.index(challenge.responder_index))
+    custody_bit = attestation.custody_bitfield[attesters.index(challenge.responder_index)]
     assert custody_bit != get_bitfield_bit(get_chunk_bits_root(challenge.chunk_bits), 0)
     # Add new bit challenge record
     new_record = CustodyBitChallengeRecord(

specs/core/1_shard-data-chains.md

@@ -92,7 +92,7 @@ class ShardAttestation(Container):
         slot: Slot
         shard: Shard
         shard_block_root: Bytes32
-    aggregation_bitfield: Bytes[PLACEHOLDER]
+    aggregation_bitfield: Bitlist[PLACEHOLDER]
     aggregate_signature: BLSSignature
 ```
 
@@ -230,10 +230,9 @@ def verify_shard_attestation_signature(state: BeaconState,
                                        attestation: ShardAttestation) -> None:
     data = attestation.data
     persistent_committee = get_persistent_committee(state, data.shard, data.slot)
-    assert verify_bitfield(attestation.aggregation_bitfield, len(persistent_committee))
     pubkeys = []
     for i, index in enumerate(persistent_committee):
-        if get_bitfield_bit(attestation.aggregation_bitfield, i) == 0b1:
+        if attestation.aggregation_bitfield[i]:
             validator = state.validators[index]
             assert is_active_validator(validator, get_current_epoch(state))
             pubkeys.append(validator.pubkey)

specs/light_client/sync_protocol.md

@@ -168,7 +168,7 @@ If a client wants to update its `finalized_header` it asks the network for a `Bl
 {
     'header': BeaconBlockHeader,
     'shard_aggregate_signature': BLSSignature,
-    'shard_bitfield': 'bytes',
+    'shard_bitfield': Bitlist[PLACEHOLDER],
     'shard_parent_block': ShardBlock,
 }
 ```
@@ -180,13 +180,13 @@ def verify_block_validity_proof(proof: BlockValidityProof, validator_memory: Val
     assert proof.shard_parent_block.beacon_chain_root == hash_tree_root(proof.header)
     committee = compute_committee(proof.header, validator_memory)
     # Verify that we have >=50% support
-    support_balance = sum([v.effective_balance for i, v in enumerate(committee) if get_bitfield_bit(proof.shard_bitfield, i) is True])
+    support_balance = sum([v.effective_balance for i, v in enumerate(committee) if proof.shard_bitfield[i]])
     total_balance = sum([v.effective_balance for i, v in enumerate(committee)])
     assert support_balance * 2 > total_balance
     # Verify shard attestations
     group_public_key = bls_aggregate_pubkeys([
         v.pubkey for v, index in enumerate(committee)
-        if get_bitfield_bit(proof.shard_bitfield, index) is True
+        if proof.shard_bitfield[index]
     ])
     assert bls_verify(
         pubkey=group_public_key,